The Small Blue is a small, very localised and easily overlooked butterfly that occurs in small discrete colonies on herb-rich grassland on calcareous soils where its foodplant Kidney Vetch (Anthyllis vulneraria) grows. It has declined throughout most of its British range. Changes in management are likely to affect the quantity and quality of foodplant available for oviposition and subsequent survival of the larvae. Some disturbance is usually necessary to provide good germination sites for the establishment of new plants of this biennial or short-lived perennial.(For further details on this species see http://www.butterfly-conservation.org/).

Log collated index plot

This chart shows the index of abundance (LCI = Log Collated Index) over time. It shows fluctuations in
populations from year to year, and is scaled so that the average index over the whole series is equal to 2
(horizontal line). For greater detail about how this index is derived, click on the green question mark above.

Trend description :This butterfly is recorded regularly on a relatively small number of transects and the data show no significant trend. This is reflected in the site level trends, with no significant declines or increases at any site with sufficient data to calulate a long-term trend. Abundance on monitored sites can be extremely large, reaching three figure indices regularly at sites such as Swyncombe Downs in Oxfordshire, where in 2005 Small Blue produced an impressive site index of 1,042, the highest recorded on any UKBMS site. It was once considered a normally single-brooded species in Britain and Ireland, but in recent years it has shown an increasing tendency to produce a second brood, presumably in response to a warming of the climate.

Phenology plot

Phenology plot

This chart shows the average number of butterflies seen on transects between Arpil and October across all sites (fitted values from a Generalised Additive Model). The blue line gives average counts over the full BMS series (1976 to date) and the red line gives the average for the last year.

This map shows symbols for the mean abundance at transect sites, with the colour of the symbol reflecting the level of abundance. Means are over all years. Grey background squares are the occupied cells as shown by the Butterflies for the New Millenium over the previous ten year period.

Coverage

In total, Small Blue has been recorded from 282 transects in the Butterfly Monitoring Scheme. Of these, annual indices of abundance have been calculated from 257 sites, with an average index of 15 individuals per site.

For 84 of these sites, Small Blue has been recorded well enough to calculate annual indices of abundance in more years, allowing trends to be calculated.

In 2017, 2714 individuals were recorded from 83 sites, producing annual indices at 69 of these.

This map shows the trend in abundance at particular transect sites for which data has been received within the last five years. Trends (increasing, declining or stable) are assessed at sites where the species has more than five years of annual index data. Use the option boxes below to view plots for individual sites.